This invention relates to improvements in a mold, especially for making the housing or an audio cassette tape or the like.
The invention is aimed at providing such a mold of long service life capable of producing a cassette housing which does not hamper smooth tape running and has a see-through window protected from damage. The mold is for use in a process for simultaneously molding materials of varied kinds or colors, or a process of injection molding whereby a unitary molded article of two or more different colors or kinds is obtained by a combination of a mold a part of which is shiftable in position to provide an additional mold cavity or cavities for secondary or further injection shots contiguous to the cavity for the first shot, and separate injection units for primary and secondary or further shots (the process being hereinafter called "stepwise molding process").
Before proceeding to the description of the present invention, an example illustrative of the conventional stepwise molding process incorporating the so-called hot-runner molding technique will be explained. The process is applied here to the manufacture of an upper housing section (of a colored resin) with a see-through window (of a transparent resin) of an audio cassette. Referring to FIG. 1 of the accompanying drawing, there is shown the cassette tape as assembled, with the housing sections 1 and 3 made of a colored resin and the window 2 made of a transparent resin.
FIGS. 2 and 3 illustrate different stages of stepwise molding process using the hot-runner molding technique for forming the upper housing section 1 and the see-through window 2 shown in cross section through the line A-A' in FIG. 1. FIG. 2 is the sectional view of the mold arrangement at the time of primary injection, or when the upper housing section 1 is made. In the figure, 4 is a fixed mold half, 5 a movable mold half, 6 a slide core, 7 a primary injection gate, and secondary injection at a secondary injection gate 8. The colored resin in molten state to form the upper housing section 1 is forced through the gate 7 to fill up the mold cavity and become solid. FIG. 3 is the sectional view of the mold arrangement at the time of secondary injection, or when the see-through window 2 is formed. On completion of the upper housing section 1 the slide core moves a distance equal to the thickness t of the see-through window 2 in the direction of the arrow. The second cavity thus formed is filled up with the transparent resin in molten state shot through the gate 8. The heat of the newly introduced transparent resin is partly conducted to the colored resin already solidified by the injection pressure, half melting the resin portions in contact with the new shot. When these portions have cooled down and solidified again, the resulting upper housing section 1 and see-through window are integrally united.
For added strength of the bond between the colored resin and the see-through window, the latter needs extra bonded portions in parallel with the horizontal planes of the upper housing section. Thus, the see-through window must have shoulders or steps as indicated in FIGS. 2 and 3. Structurally, this results in the formation of a recess 9 on the inner side of the see-through window, with a depth equal to the height of the steps d. This recess can sometimes cause troubles in the case of an audio cassette or the like which has specified internal dimensions and uses sheets interposed between the tape wound therein and the upper and lower housing sections. The existence of the above recess makes if difficult for such a sheet to press the tape and regulate its winding or unwinding motion, while allowing the sheet to slacken out of shape within the recess, leading to unsmooth running of the tape.
Therefore, it is an object of the present invention to provide a mold capable of avoiding the formation of a recess on the inner side of a secondarily formed section during the secondary injection of a stepwise molding process.
Another object of the invention is to provide a mold of the character defined above which exhibits long service life without being damaged on its molding surfaces.